This invention concerns a method and a device for recycling exhaust gas recycling gasses to a combustion engine intake and particularly to handling exhaust gas from two exhaust collectors. It also concerns a supercharged diesel including such a device.
In turbo-supercharged diesel engines it is previously known to recycle exhaust gases to the engine inlet in order to reduce contents of nitrous oxides in the exhaust gases. Hereby the recycled exhaust gases function so as to lower the combustion temperature resulting in that a smaller amount of the nitrogen in the inlet air can be converted into nitrogen oxides. This process, usually called EGR (exhaust gas recirculation) has often been used in Otto engines as a relatively simple way of reducing the contents of harmful exhaust gas emissions. In diesel engines, on the other hand, this technique has not been employed to such a great extent depending i.a. on the fact that there are particular problems associated with these engines, making Otto engine solution not directly applicable in diesel engines.
One of these particular problems is that the combustion in diesel engines normally occurs with excess air. This indirectly results in need of transferring relatively large amounts of exhaust gases during a relatively large operating range of the engine in order to achieve the desired function. This problem is accentuated in case of an engine of the supercharged type, because in that case the pressure in the intake system of the engine is greater than the pressure in the exhaust gas system during a great part of the operating range.
Among known solutions to be used in supercharged engines, two main principle solutions can be distinguished, usually named xe2x80x9cshort route EGRxe2x80x9d and xe2x80x9clong route EGRxe2x80x9d. In the first mentioned case exhaust gases are taken from a position before an exhaust turbine in the exhaust system and is recycled to a position after an intake air compressor which is arranged in the intake system. In the latter case exhaust gases are taken from a position after the exhaust turbine and are recycled to a position before the intake air compressor. Both of these principle solutions have advantages and disadvantages.
Also U.S. Pat. No. 5,611,203 and U.S. Pat. No. 5,611,204 could be mentioned as previously known art with respect to this invention. These documents describe how exhaust gases are recycled to the intake in turbo-supercharged diesel engines through a venturi device or any other kind of ejector being placed in the intake channel. The system according to these documents uses the low static pressure prevailing in a certain section of the ejector device for pumping-in an EGR flow into the charged air.
In supercharged diesel engines having double exhaust collectors, some times only exhaust gases from one of these exhaust collectors are used as an EGR source which however results in an uneven EGR flow which in turn may affect the engine so that a correspondingly uneven operation will result.
In previously known devices where both exhaust collectors are being used as an EGR source indeed a more even EGR flow has been obtained, but at the fusion of the gas streams, a cross flow easily occurs from that channel where, at the moment, the highest pressure prevails, to the channel where the lower pressure prevails. This means that taken together, a lower pressure of accessible EGR gases will result and thus an increased power need in order to pump them into the intake channel of the engine.
There is also a risk for a negative effect on the turbo operation, since available energy may be reduced.
In order to avoid these problems it has been suggested to provide one-way valves in the channels up-stream the point of fusion, but this results in an unwanted pressure drop and lower reliability of operation and increased costs.
It is an aim of this invention to provide a solution to or a reduction of the problems of the prior art. A main aim of the invention is thus to provide a simple, long life, economic and effective solution for EGR transfer.
According to the invention this is achieved in a method and a device having a mixing section for mixing gasses from two exhaust collectors using, in the flow direction, a contraction section for each flow from an exhaust collector, followed by a common transition section followed by an enlarged diffuser section.
Through the invention it is achieved that EGR gases are accelerated in the contraction portion, resulting in an increase of the dynamic pressure during simultaneous reduction of the static pressure. By designing the contraction portion such that the accelerated EGR gases from the exhaust collector with the highest prevailing pressure are accelerated so that their static pressure correspond to the static pressure of the EGR gases from the exhaust gas collector with the lowest pressure, at the moment, no cross flow will occur and it will be secure to combine the channels. The leak flow between the collectors thereby has been eliminated through solely flow modifying measures and without any use of valves of the Reed type or the like. Further, a back flow from the upstream side is avoided.
Further, the transition portion is constructed such that the gas stream from that part of the contraction portion, where at the moment the highest pressure prevails, easily goes over to and adjoins with the xe2x80x9coppositexe2x80x9d side during simultaneous continuous adjoining to its xe2x80x9cownxe2x80x9d side.
Further, the diffuser portion is constructed such that the gas stream continues to adhere and adjoin to the wall of the diffuser portion over its entire length without any vortex formation occurring through diversion of flow. This is obtained by testing and dimensioning of the different portions of the mixing section while considering for example the temperature gradient and the density gradient of the gas as seen in the length direction of the mixing section. As an example it could be mentioned that the cross-section of the transition portion is preferably elongated, essentially rectangular, or at least almost slot-shaped, with the dimensions, as an example for a six cylinder diesel engine having a cylinder volume of about 11 liters, ca 12 mmxc3x9725 mm.
By shaping the wall separating the parts of the contraction portion as a plate having its down stream edge ended with a sharp edge or sharply cut-off edge, simple production and effective function is achieved.
Further features and advantages of the invention will come clear from the following description of embodiments.